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Re: [TowerTalk] Stacking Dissimilar Antennas

Subject: Re: [TowerTalk] Stacking Dissimilar Antennas
From: Pete Smith <>
Date: Mon, 21 Aug 2006 07:03:19 -0400
List-post: <>
Just one added point - for tri-bander stacks, spacings of less than .5 
wavelength on 20M are quite common because the spacing used is necessarily a 
compromise between three bands (or at least 2, these days).  Fortunately, 
calculating maximum forward gain is not what we use HFTA for anyhow.

73, Pete N4ZR

  At 01:17 AM 8/21/2006, K4SAV wrote:
>I need to correct a couple of statement I made about HFTA.  I said HFTA 
>always gives 3 dB gain for any antenna stacking distance.  That is not 
>correct. It gives 3 dB gain for very closely stacked antennas, less than 
>about 0.5 wavelengths.  For other wider spacings it seems to function 
>correctly. Gains for closely spaced stacks, can be manually corrected 
>with data from NEC, but you will discover that anything less than about 
>0.4 wavelengths yields so little gain as to not be worth the effort of 
>stacking. N6BV recommends stacking no less than 0.5 wavelength for HFTA 
>because of the gain errors produced.
>Also the gain error is because of the math models used for the stacking, 
>not the ray tracing algorithm.
>Jerry, K4SAV
>K4SAV wrote:
>>Be careful when using HFTA to model stacks.  HFTA does not model antenna 
>>interaction.  All rays are assumed to go outward from the antenna 
>>between 0 and 34 degrees elevation.  No rays directed upward at the 
>>other antenna are modeled. So HFTA will always give 3 dB gain for ANY 
>>stacking distance. This is not the way it works in the real world. NEC 
>>will show the correct amount of gain from the stack.
>>The big advantage of HFTA is that it can handle irregular terrains where 
>>NEC cannot. Your ground can also affect the optimum stacking distance. 
>>In other words, an optimum stack over flat ground is not necessarily an 
>>optimum stack over sloping ground. You can use HFTA results up until you 
>>get to the point where you start to loose gain because of a stacking 
>>distance which is too close.  You can determine this distance using NEC. 
>>If you do not have flat ground, using HFTA is to evaluate the stack is a 
>>good plan, because in some cases the terrain can have a huge affect. If 
>>you go closer than that stacking distance, calculated by NEC which 
>>starts to decrease the gain, you can manually correct HFTA gain numbers.
>>Jerry, K4SAV
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